Vacuum tube with high-emission cathode



Patented Aug. 4, 1936 UNETED gAS VACUUM TUBE WITH HIGH-EMISSION CATHODE Siegmund Loewe, Berlin, Germany No Drawing. Application September '14, 1932, Serial No. 633,080. In Germany September 15,

The invention relates to a high-emission cathode for vacuum tubes, for example rectifiers, amplifiers, oscillation generators. Originally there were employed so-called smear oxides, which were painted onto the filament. On occasion the oxide coating was also applied electrolytically.

A much more satisfactory and already known method consists in applying a light metal to the filament by evaporation, whereby oxidation takes place subsequently (metal vapour process). The cathodes produced in accordance with the metal vapour process are excellent as regards efiiciency; from the standpoint of manufacture, however, the process is not'an entirely simple one.

The invention relates to a high-emission tube having an oxide cathode, but consists of an additional development of the cathode production on such lines that the emissive oxide, for example light metal oxide, is first produced on the carrier filament, preferably according to the evaporation process, or according to an electrolytical process. The carrier filament coated with the metal oxide is then fitted as cathode in the discharge vessel, and is there raised to high emission by means of a special treatment. This special treatment consists of two measures, viz., in the first place in a heating of the filament in the absence of air, which causes initially a degasifying of the filament and, upon additional heating, a conversion of the carbonate and hydroxide, which are formed in addition to the oxide, into metal oxide; secondly in application, by atomization, of a light metal onto the pure metal oxide which is now present on the filament in the evacuated tube.

Certain of the measures described in the above are old, for example the heating of a carrier filament, which is coated with metal oxide, in vacuum for the purpose of complete conversion of any carbonate and hydroxide present into metal oxide.

The invention, however, resides in the combination of various features, which only by reason of their particular cooperation are adapted to produce high-emission cathodes of equal effectiveness throughout, such as are capable of being produced according to the metal vapour process, but by a technically more simple method. The point concerned is the common use of the following factors:

(A) The application of the layer of oxide to the carrier filament according to a process which ensures a perfectly even thickness of layer and also structure of the layer;

(B) Conversion of the hydroxide and carbon- 1 Claim. (01. 250-275) ate, which are present, upon the assembly, in the filament preliminarily treated in accordance with (A), into pure metal oxide, following expulsion of the gas;

(C) Application of a surface deposit of light metal onto the cathode preliminarily treated in accordance with (B), for the purpose of initiating the emission.

As regards point (A) it may be remarked as follows: The appreciable increase in emission, which is obtained by cathodes produced according to the metal vapour process as compared with other methods must be maintained. For this purpose the filament material is coated in relatively large quantities, for example in loosely wound wire coils, with a layer containing oxygen on the surface. 'The most desirable embodiment is constituted by a thin layer of brown tungsten dioxide on tungsten wire. The layer may be produced in continuous fashion at certain temperature in a furnace, which is filled with hydrogen having a certain percentage of water. The filament coated with tungsten dioxide is now introduced in a relatively large quantity, for example in the form of a loose roll, into a vacuum furnace, in which there is created a barium-vapour atmosphere. With suitable conditions of temperature the metallic barium vapour reacts with the tungsten dioxide in the known manner, in such fashion, that on the surface of the tungsten wire there is now formed barium oxide, in which connection, however, there is also an excess of metallic barium present. The excess of metallic barium, according to the invention, is again pumped away or bound otherwise in the furnace, for example is condensed by cooling, so that in the furnace there now remains the tungsten wire coated with pure barium oxide without an excess of metallic barium.

The above represents the feature (A) of the combination in the invention.

Re: (B). If the preliminarily treated wire is now removed from the vacuum furnace, the barium oxide on the filament changes in the course of time into hydroxide and carbonate. The material may nevertheless continue to be worked up technically without objection, for example cut into pieces and fitted in the form of cathode into discharge vessels. The discharge vessel is now completed and furnished with means, which permit the metal barium vapour to develop during the pumping process. The discharge vessel is now pumped in the usual fashion, in which connection, however, by heating the filament, at

first a degasifying of the same is caused, and by additional heating complete conversion of the barium hydroxide and carbonate formed into barium oxide. This is feature (B) of the combination, whereby it may be remarked as follows: Without the feature (C) set forth in the following a cathode preliminarily treated in this fashion would not result in any kind of emission, except possibly on account of incompletely expelled traces of gas or other sources of disturbance. This emission would at least not be steady, and would be much below the value which the same would be able to acquire in the case of complete production of the cathode. To obtain a product of great durability and output there is required the third feature.

Re: (C). In the discharge vessel a light metal is caused to evaporate in the manner known from the metal vapour process, viz., either the same metal from which the metal oxide is formed (in the described example: barium) or another metal, preferably a light metal with higher evaporation point than the metal base of the oxide employed. However, it is also quite possible to employ the same metal or one with lower evaporation point. The metal coating applied by atomization has the surprising effect of initiating the emission process, so that in discharge vessels of this nature completion may be performed without the presence of any kind of gas. As well known, this otherwise is only possible in the case of highemission cathodes on the lines of the tungstenthorium cathode, or in the case of oxide cathode tubes produced according to the metal vapour process in its previously known form. The invention, therefore, resides in the production of discharge tubes having high-emission cathodes, which possess all of the advantages of the metal vapour process, and also the additional advantage that the production of the light metal oxide may take place outside of the tube without necessity for the presence of residual gas for the purpose of forming the cathodes. It is shown that cathodes produced by the method according to the invention even result in an improved efliciency as compared with cathodes produced according to the pure metal vapour process, apparently owing to the fact, that control of the chemical reaction (conversion of the tungsten dioxide into barium oxide) can be performed more effectively in a special vacuum furnace than in the actual discharge vessel, in which it is difficult to select all of the conditions of reaction in such reliable fashion as in a special vacuum furnace.

It may also be remarked that the binding of the excess of metal vapour in the vacuum furnace is compelled to take place for the reason that if an excess of the metal vapour required for the reaction is allowed to remain, uncontrollable variations take place in the thickness of the layer after the opening of the furnace, which variations result in disturbances in the uniform nature of the emission.

It is furthermore remarked that the production of uniform coatings of metal oxide on the carrier filament, in addition to the manner described (viz., in a suitable vacuum furnace), is also possible in electrolytical fashion. experience has shown that smear oxides are not adapted for the purpose of the invention. Of the two kinds first mentioned (metal vapour and electrolytical process) the method described, however, is to be given preference.

I claim:

A method of producing highly emissive cathodes comprising the steps of coating a preferably loosely wound tungsten wire roll with a thin layer of brown tungsten dioxide by heating said wire in a furnace filled with hydrogen having a certain percentage of water, introducing said oxidized wire into a vacuum furnace filled with a barium-vapor atmosphere and heating said wire for forming barium oxide on the surface of said wire by reaction of said barium vapour with said tungsten dioxide maintaining this process until the reaction is performed on all parts of said wire removing the excess of metallic barium, cutting said wire roll into pieces, fitting said pieces in the form of cathode filaments into discharge vessels, heating said filaments at first for degasifying and then heating further for completely forming barium hydroxide and carbonate produced on the surface of the wire by reaction with the atmosphere into barium oxide.

SIEGMUND LOEWE.

On the other hand 

